[mmox] LLSD and content schema

[Jon Watte <[e-mail]>]

Meadhbh Hamrick (Infinity) wrote:
> * LLSD is implementable by mortals in a tractable amount of time and 
> without the use of complicated third-party tools (ASN.1 compilers, etc.)

I don't see the LLSD interface compiler as any different in conceptual 
complexity than an ASN.1 compiler. I did bring up the question of ASN.1 
before, and was told by you that you felt it was "too buggy," which I 
think is not fair or accurate.

However, I believe ASN.1 and also LLSD have a problem that I'd like to 
see addressed in a standard: they use implicit schema.

If I want to send a piece of data from point A to point B, and the 
points don't both exactly know what the data is, that's fine -- but, in 
that case, you typically want to use something a little more open-ended 
and more generally standardized, like XML. The benefit of XML is that 
there are tons of tools that already support it (as there are with 
ASN.1, btw); XML also allows well-formed documents to be verified using 
XSD schema. If you need large binary blob support (textures, etc), those 
can be included by reference uri. I think XML is appropriate for those 
cases, and LLSD doesn't quite fit the bill in that case.

If I want to send a piece of data from point A to point B, and point A 
and point B know exactly what this data will be, then data type 
information does not need to be sent as part of the serialization. In 
fact, including data type information is redundant and consumes network 
bandwidth unnecessarily.

Thus, I would assume that any communication where size matters (such as 
for entity property update streams) will be done using negotiated 
(bilaterally understood) schema, with the following properties:
1) Only data elements, not names or types, are sent.
2) Specfic properties are addressed using short Ids that are translated 
in schema.
3) If certain properties are always sent together (say, position x y z 
and orientation x y z w) then schema allows for an "aggregate property" 
to be declared, such that only a single name (Id) is needed to identify 
that entire tuple.


I have written an illustrative proposal up for this that may help 
clarify the issue, at the following URLs:
http://www.interopworld.com/node/29
http://www.interopworld.com/node/28

Sincerely,

jw